Repositori URV

Identifier: TDX:761

Handle: https://hdl.handle.net/20.500.11797/TDX761

Authors:
Viso Acosta, Antonio

Abstract:
In the present PhD-thesis doctoral titled 'Stereoselective synthesis of 2'-desoxi-, acyclo, and isonucleosides' new methods have been developed to obtain nucleosides. The work has been divided in three sections sumarized below: Firstly 2-deoxy-2-sulfenyl-arabino--nucleosides and 2-deoxy-ribo--nucleosides were obtained with excellent stereoselectivity from 1-thio-ribo- and 1-thio-arabino-glycosides in a one pot reaction under Mitsunobu conditions. The stereoselectivity depends on the substituent bonded to the sulfur and on the reaction conditions, particulary the solvent. From the results obtained it can be concluded that the reaction intermediate is an oxonium cation and not the thiiranium cation. DFT calculations for determining the geometry of reaction intermediates in glycosilation reactions support an average structure between oxonium and thiiranium cation for tetrahydrofurane and tetrahydrothiofene derivatives. This is the result of a weak interaction between the heteroatom and C-1, which helps to stabilize the cation. Electron donor substituents bonded to external heteroatom (R in SR and SeR) or electronwithdrawing groups bonded to O-5, produce an important change in the geometry: the reaction intermediate becomes more like an episulfonium, episelenonium or iodonium cation. The structure of tetrahydropyrrole derivatives is iminium-like. This fact agrees with the electrondonor properties of nitrogen, which are better than those of oxygen and sulfur. When strong electronwithdrawing groups are bound to the endocyclic heteroatom, or they do not have an electron pair, their structures are episulfonium, episelenonium- and iodonium-like cations. Obviously, this is also the case for the carbocyclic derivatives.Next, enantiopure 2,5-dihydrofuran derivati